Embryonic, neo- and postnatal heart development will be studied in cardiomyopathic (UM-X 7.1) and age-matched normal hamsters, with two aims: (1) Because of inconsistent previous results, it is necessary to establish unequivocally by morphological examination whether differences arise during development in (a) heart chamber form, (b) heart wall thickness, (c) contractile cell density, (d) level of cardiac differentiation, i.e., whether the cardiomyophathic heart is differentially retarded or advanced over normal, or hypertrophied. Unbioased, highly reproducible stereological techniques employing point counting and line integration will be sued at both the light microscope and EM levels; (2) Because catecholamines and calcium seem to trigger the myolytic, necrotizing heart lesions which appear in the first month of life, whole fetal and neonatal hearts will be organ-cultured, their levels of contractility (max dF/dt) established using sensitive techniques for small hearts, and experiments carried out to test the effects (on contractility and heart rate) of calcium, catecholamines, and various substances which modify their cardiac effects. Hopefully, this study will help clarify the basic defect which results in 100% death of UM-X 7.1 hamsters from heart failure; it should help fill a gap in present knowledge, because this excellent animal model of inherited cardimyopathy has been studied very little in its "pre-clinical" stages.